In order to form a neat and strong weld at a seam between two workpieces it is standard as described in U.S. Pat. No. 5,126,526 of Schnorrer and in my U.S. Pat. No. 5,235,152 to juxtapose with one side of the seam a so-called hindgas implement or device. This device forms a generally closed chamber that extends along the back of the seam and that is flooded with an inert gas so that the surface of the weld does not oxidize. In order to protect the appliance, it is normally made of a ceramic that can withstand the heat of the welding operation.
In a standard such device as described in "Der Praktiker" (July 1995, pages 330-332) a row of ceramic blocks form several laterally closed passages through which extend flexible metallic wires so that the device can be shaped to conform to the shape of the workpieces. Such a device is fairly difficult to fit accurately to the workpieces, especially when the seam is curved, and is also very hard to hold securely in place during the welding operation. Normally suction or magnetic clips are used to hold the device in place, making its installation fairly difficult, particularly inside a pipe.
Another known system uses sticky tape to secure the ceramic bodies in place during the welding operation. While fairly simple, the adhesive often lets go when heated by the welding operation, and the blocks are not held solidly enough that the chamber between them and the workpieces can be pressurized or at least effectively flooded with the protective hindgas. When the adhesive is heated it can produce vapors that damage the weld.